In digital circuits and systems, a collection of logic gates that performs Boolean or logic functions on input signals to create output signals is commonly referred to as a combinational logic circuit. The basic building blocks of a combinational logic circuit are NOT, NOR, and NAND logic gates. Additionally logic gates such as OR, AND, and XOR logic gates may be constructed using these basic building blocks.
At a physical level these logic gates are formed from transistors. In particular, complementary paired transistors are configured in multiple types of configurations in order to create specific logic gates. Because transistors are made of semiconductor materials that do not withstand ions transitioning through them without a change in operating state, radiation events (e.g., particle strikes) may cause one or more transistors within a logic gate to become conductive and change their output states. A radiation event, also referred to as a glitch, may initiate logical switching in a logic circuit which may result in two basic effects: a Single Event Transient (SET) or a Single Event Upset (SEU). Typically, within the duration of a glitch, a disturbed transistor will recover back to its previous operating state unless its control voltage level has been affected by the glitch.
The first effect, a SET, is a glitch logically propagated from an affected node to a logic circuit output. If such a glitch causes a change in state of a memory circuit then this effect becomes the second type of effect: an SEU or soft error. SEU events may be detrimental to a memory circuit and circuits relying on the memory circuit. In addition, the wrong output signal at the data output of a combinatorial logic circuit could cause circuits relying on the combinatorial logic circuit to malfunction or be delayed.
One method of reducing radiation effects on a logic circuit is used to implement a majority voting scheme. A sample majority voting scheme 20 is shown in FIG. 1A, which includes three redundant logic circuits 22, 24, and 26 each receiving a same logic input 21. A respective output 23, 25, and 27 from each redundant logic circuit 22, 24, and 26 is then fed to a voting circuit 28. If a radiation event occurs, such as an SET, combinatorial logic within the voting circuit 28 is used to determine a correct output based on the “majority” of signal levels it receives. For example, if a radiation event occurs on one of the redundant logic circuits 22, 24, and 26, one of the outputs 23, 25, and 27 of one of the redundant logic circuits 22, 24, and 26 will be invalid. Because the other outputs of the redundant logic circuits 22, 24, and 26 should have a correct output, however, voting circuit 28 will provide a correct output 29 having a logic level matching that of the majority of the outputs 23, 25, and 27 of the redundant logic circuits 22, 24, and 26.
While this does provide efficient protection against radiation events, it consumes an undesirable amount of space, and may consume an undesirable amount of power.
Another option for protection against radiation events is shown in FIG. 1B. Here, rather than use redundant logic circuits and a voting circuit, filtering is used. In particular, the radiation protection circuit 30 includes a logic circuit 32 receiving an input and producing a logic output 33. An RC filter 34 receives the logic output 33 from the logic circuit 32 and has a time constant sufficient such that brief transients (a logic level different than that intended) in the logic output 33 are filtered out. The filtered output 35 provided by the RC filter 34 is therefore accurate. However, due to the time constant (which is relatively large), significant propagation delay is introduced from the logic output 33 to the output 35, limiting the potential operating frequency. Therefore, this radiation hardening circuit 30 is unsuitable for certain operations, such as generating pulse width modulation control signals.
As a consequence, further development is required.